The present invention relates generally to fail-safe actuator mechanisms and more particularly concerns a fail-safe actuator which automatically returns a fluid media controlled valving member to a predetermined position whenever the pressure of the controlling fluid media falls below a predetermined level.
In many situations, it is imperative that a valve member is returned to a predetermined position in the event of a power failure. One highly successful device for insuring the return of such a valving member is shown in applicant's U.S. Pat. No. 3,752,041. This mentioned patent discloses a fluid operated vane actuator with control valve means connected between a source of pressurized fluid and a first side of the actuator. A by-pass conduit also communicates with an accumulator tank through a restrictive orifice, and the accumulator tank, in turn, communicates with the second side of the actuator. Whenever a mechanical or electrical failure causes a sudden loss of air pressure, the restrictive orifice produces a disparate flow rate between the accumulator tank and the first side of the actuator on one hand, and the actuator tank and the second side of the actuator on the other hand, the flow to the second side being far greater. Due to the disparate flow rates, the actuator is urged toward the first side and the controlled valve is moved to a closed position.
Another highly successful fail-safe actuator is disclosed in applicant's co-pending application Ser. No. 944,771 filed Sept. 22, 1978, now U.S. Pat. No. 4,275,642. This apparatus discloses an actuator which is entirely enclosed within a steel encapsulating housing to prevent exposure of the actuator to potentially harmful vapors. Pressurized fluid mdeia is communicated directly to a first side of the actuator and indirectly to a second side of the actuator through the interior of the encapsulating member. The interior of the encapsulating member communicates with the second side of the actuator. The steel encapsulating housing thus functions both as a means to insolate the actuator from its environment and as an accumulator tank. Whenever pressure of the fluid media drops suddenly, pressurized fluid stored in the encapsulating housing flows into the second side of the actuator to move a controlled valving member to a closed position.
Frequently, fluid pressure operated systems are designed to completely shut off whenever the pressure of the operating fluid falls below a predetermined level. In some instances, however, it is desirable to use a fail-safe valve which returns a valving member to a predetermined position in a system which does not have a shut off pressure level. Since the above mentioned fail-safe devices are designed to operate in response to sudden pressure decreases, they are ineffective in situations in which the fluid pressure "sags" or drops slowly.
Moreover, it is sometimes desirable to cycle a fail-safe valving member repeatedly within a short period of time. Balanced air systems, like those of the fail-safe actuators discussed above, have time limitations for repeated cycling.